Inverse emulsions (i.e., water-in-oil emulsions) are commonly used to prepare high molecular weight water-soluble and water-swellable polymers. The inverse emulsion medium allows liquid, flowable polymer compositions to be prepared, which when added to water in the presence of an appropriate surfactant, will “invert” to form an aqueous solution or an oil-in-water emulsion. Inverse emulsion polymerization is used in these cases because direct polymerization in bulk water solution would generally lead to unacceptably high viscosities, undesired gelling, in efficient mixing, and other problems associated with very high molecular weight water soluble polymers. These problems can be avoided in the inverse emulsion medium, because the high viscosity aqueous phase that results from the polymerization reaction is dispersed as droplets in a relatively low viscosity oil phase, maintaining fluidity of the bulk emulsion.
Uncontrolled polymerizations, i.e., “runaway” reactions, are a constant and dreaded concern to producers of inverse emulsion polymers. Runaways can lead to catastrophic breaking of the emulsion and formation of an intractable gel/bulk polymer mass in the polymerization reactor, or to formation of large quantities of insoluble and soluble gel suspended in the oil phase, rendering the entire product worthless. Neither outcome is desirable. Reactors can become so fouled that complete cleaning is not possible or practical, and the reactor must be scrapped. The gel-containing runway products not only waste the costs of starting materials and labor involved in the production process, but also add costs for disposal of the unusable product.
In order to avoid runway inverse emulsion polymerization reactions, sophisticated reaction temperature control systems, incremental initiator feeds, higher speed agitation, and other expensive expedients must be utilized during convention inverse emulsion polymerization processes. When such controls are implemented, reaction times for the polymerization reactions can increase to several hours. Fundamentally, free radical polymerization reactions in the small aqueous droplets present in the inverse emulsions have the potential for much shorted reaction times, were it not for all of the control features that must be implemented, which effectively slow the reaction. Because of the many difficulties discussed above, there is an ongoing need for new inverse emulsion polymerization processes that can be efficiently carried out without sophisticated and expensive reaction controls. The methods described herein address this need.